The present invention is directed to weight measurement, and more particularly is directed to a method of scale calibration and weighing, using an electronic scale.
Weighting scales are widely used in supermarkets or the like for weighing produce or other merchandise items and must meet stringent requirements as to performance and cost. The scales must be accurate enough to satisfy public weights and measures authorities. These requirements are usually met at the time that the scale is manufactured by an initial adjustment. As part of these requirements, the scale must display a predetermined reference value, normally zero, when no merchandise item is begin weighed so that the change in the output display produced by a merchandise item being weighed will accurately represent the true weight of the item. Through use, the zero reference of a scale may change due to a number of causes, including the following: accumulation of foreign matter on the scale platform or its supports; a change in the position of the scale platform; the effect of temperature on the load cells and electronic components; and hysteresis, especially after weighing of a heavy object. In certain types of electric scales which employ a plurality, typically four, of strain gauges, as a means of measuring the weight, these strain gauges are usually configured in a Wheatstone bridge circuit for increased accuracy because their fractional change in resistance when strained by a force is very small (for metal-wire strain gauges, typically of the order of 0.1%). The signal outputs from the strain gauges in the Wheatstone bridge circuit are summed algebraically to provide a signal representative of the weight being determined. In such precision scale applications, the strain gauges must have precisely equal sensitivities, i.e. the same fractional change in resistance when strained by the same force centered at the same relative position. In at least one type of scale, this achievement of equal sensitivities is accomplished by precisely sanding the bases of the strain gauges, a slow, expensive, labor-intensive method.